Future generation mobile networks and other cellular communication systems will generally require a higher density of radio base stations and higher data rates compared to previous network generations. An efficient way of addressing this problem is to distribute remote radio units of relatively low complexity at suitable locations over a given network area and interconnect the remote radio units to a central main unit using optical fiber. This main-remote concept, which is sometimes referred to as Fiber-To-The-Antenna (FTTA), gives a virtually unlimited data rate between the main unit and the remote radio units. The main-remote concept provides a very cost-effective way of distributing radio heads if fiber already exists or can be deployed easily. In urbanized areas, however, it is often difficult and expensive to install or rent fiber. The waiting time for deploying fiber can also be considerable. The latter may be a crucial factor for operators, which may loose market shares and even be subjected to order of punishment if the deployment is not completed in time.
Free-space optics is an interesting alternative to fiber that can be used for providing cost-effective point-to-point communication. The main advantage of using free-space optical communication links is that hard-wired connections are avoided. Free-space optical links are easily deployed and may provide data rates comparable to those of optical fibers.
U.S. Pat. No. 5,844,705 describes a communication system employing free-space optical links. A conventional cell in a wireless communication system is divided into a number of sub-cells, each of which is served by a respective radio antenna. Each sub-cell antenna includes an optical transceiver that communicates with the central unit, formerly serving the overall cell, via a respective free-space optical link. The central unit is converted from a radio frequency transceiver to a set of optical transceivers for optical communication with the sub-cell transceivers.
A major source of concern, however, is the availability of the free-space links, or wireless or fiberless optical links as they are often called, since the link quality is highly dependent on the atmospheric conditions. For example, it is well known that wireless optical links are sensitive to snow, fog, smoke and dust. One way of reducing the adverse influences of so-called atmospheric attenuation is to use laser beam transmissions at frequencies allowing greater penetration and less absorption or scattering by atmospheric influences.
U.S. Pat. No. 6,239,888 describes an all-optical integration of optical fibers and wireless optical links by means of erbium-doped fiber amplifiers. Erbium-doped fiber amplifiers are generally capable of providing all-optical, broadband amplification at optical frequencies compatible with those used in optical fibers.
Other approaches for addressing the availability problem of wireless optical links include multi-beam configurations and microwave back-up to provide secure point-to-point communication.
All these approaches have two things in common—they focus on increasing the link availability and they are expensive.
U.S. Pat. No. 6,314,163 describes a hierarchical architecture with picocells interconnected by free-space optical links, with a standard cell providing alternate back-up access.